Federal, state and local governments have recognized a correlation between fish kills and gas supersaturation at dams, rivers, lakes and streams. Therefore, mandatory total dissolved gas levels have been set. Control of gas supersaturation can benefit the fisheries industry and protect the aquatic environment. Although a number of methods of controlling gas supersaturation are known, these methods suffer various disadvantages as discussed below, and a reasonably low-cost treatment is needed to control gas supersaturation at such dams, rivers, lakes and streams.
One known method of controlling gas supersaturation is dam modification. Dam designers can control variables such as shape of hydraulic structures, rates of discharge and methods of discharge to alleviate the gas supersaturation problem. For example, the Army Corps of Engineers is constructing "flip-lips" on dam spillways to control the rates of discharge and methods of discharge. The major disadvantage of this technique is that such dam modifications involve considerable cost.
Another method of controlling gas supersaturation is gas sparging. Excess air can be removed from water by sparging with a less soluble gas such as helium. The major disadvantage of this technique is the expense of helium or whatever other gas is used, particularly when large quantities of water are treated.
Another method of controlling gas supersaturation is vacuum degassing. Excess air can be removed from water by allowing the water to spill into tanks under reduced pressure, thereby driving the air from the water out into the gas phase. The major disadvantages of this technique are the costs associated with obtaining the necessary equipment, modifications needed to an existing site, and the electric power required to run a vacuum pump.
Another method of controlling gas supersaturation is turbulent aeration. Excess air can be removed from water by, for example, allowing the water to flow into rocks. The splashing action increases the air-to-water interface area, thereby causing the excess air to leave the system. This method can be used to control gas supersaturation in a river, for example. A major disadvantage of this method is the engineering difficulty involved. Moreover, in some cases, such a system may actually add air to the water. Other disadvantages of this method include navigational problems on a river presented by the materials added to the river to produce the splashing action.
Currently used methods of degassing a body of water include packed towers, diffused aeration, spray nozzles, tray aerators and cascade air stripping. In general, these methods are used to remove a gas or contaminant from water or to aerate water. Diffused aeration is of particular interest insofar as the present invention is concerned. This technology involves bringing air bubbles in contact with a volume of water so that the bubbles pass through the volume of water, collecting excess gas. Conventional diffused aeration employs large air bubbles, typically on the order of 10.sup.-2 m in diameter. The main disadvantages of this technology are the need for large volumes of air due to the large bubble size and the long reaction times required. Because large volumes of air are required, diffused aeration is unattractive for degassing large volumes of water. Because long reaction times are needed, the method defeats the purpose of degassing the water in a short time period in order to prevent damage to aquatic organisms.